In the following, some network elements important for the background of the invention are described by referring to FIG. 1.
In FIG. 1, two different networks are shown, an IP based network and a circuit switched network SCN like a public switched telephone network (PSTN) or a GSM network. These networks are connected by gateways GW1 and GW2, which are connected to switch devices SD1 and SD2, respectively. The switch devices SD1, SD2 and SD3 are conventional switches and serve to operate the SCN network. In detail, exchange terminals for a trunk line are connected to the switch devices. It is to be noted that for simplifying the illustration only an exchange terminal ET connected to the switch device SD3 is shown. Nevertheless, also the other switch devices SD1 and SD2 comprise exchange terminals.
Furthermore, an important network element for the IP network is a so-called gatekeeper. In FIG. 1, a gatekeeper GK connected to the IP network is shown as an example. In general, a gatekeeper is an element in a network, that is responsible for registration, admission and status (RAS) of terminals and gateways. The gatekeeper participates in zone management, call processing and call signalling according to the ITU-T recommendation H.323 and provides address translation. Thus, the gatekeeper determines the route for signalling and media transport (data transport) of call through the network. In this respect, the gatekeeper knows and controls the state of the IP telephony calls in an analogous way to a normal telecom switch (e.g., the switch device SD3) for a switched circuit network (SCN), like GSM or a normal PSTN. The gatekeeper GK normally handles IP telephony calls by using IP signalling, for example H.323 signalling or other relevant IP signallings like SIP (Session Initiation Protocol).
To the IP network a terminal TE is connected, which can be a computer with a telephone connected thereto, for example.
In the following, the gateways GW1 and GW2 connected to the switch devices SD1 and SD2, respectively, are described in more detail. A gateway provides an interface between two different networks, in the present case, between the SCN network and the IP network. Usually, a gateway is composed of a signalling gateway, a media gateway and a media gateway controller.
The signalling gateway provides the signalling mediation function between the IP domain and the SCN domain. It may support functional or signalling mediation between the IP domain (e.g., H.323) and call signalling in the SCN domain (e.g., channel associated signalling or non-channel associated signalling).
The media gateway provides media mapping and/or transcoding functions. It maps (e.g., tandemfree operation) or transcodes the media in the IP domain (media transported over IP) and the media in the SCN domain (e.g., PCM encoded voice, GSM, etc.).
The media gateway controller is located between the media gateway, the signalling gateway and the gatekeeper. It provides the call processing (call handling) function for the gateway. It controls the media gateways, it receives SCN signalling information from the signalling gateway and IP signalling from the gatekeeper.
Thus, the conventional gateway passes telephone traffic from the SCN network to the IP network by using IP telephony signalling used in the IP based telephony.
For this, it is necessary to provide the switch devices for special interfaces which are adapted to connect the gateways to the switch device. This results in a complicated handling of the gateways.
Furthermore, it is necessary to provide the switch devices with special control elements related to the gateways, which are different to the normal exchange terminals. Hence, a complicated hardware structure of the switch devices are necessary.